In recent years, there has been an increase in the use of composite materials for forming structural components. In high tech industries, such as the aerospace and automotive industries, composite materials have become commonplace features in just about all new designs.
Composite materials include a matrix of orientated fibrous material, such as graphite, glass and polyamide (e.g. KEVLAR), impregnated with an epoxy, polymeric, phenolic or other similar organic resinous material. (KEVLAR is a registered trademark of E.I. du Pont De Nemours and Company, Wilmington, Del.) Composite components are made from a series of composite layers or "plies" cut to a predetermined shape.
There are various processes for forming composite structures. One process is a "wet resin" process wherein dry fibers are placed within a mold and wet resin is channeled into the mold. The intermixed material is then cured to form the composite article.
A second type of process is a "prepreg" process wherein fibers are pre-impregnated with resin to form the plies. These plies are then placed in series into a mold to form a laminate, and cured to form the composite article. Typically, the plies are formed by impregnating woven fibrous cloth, yarn, or fiber tow with a predetermined amount of organic resin. The ply is then dried to form a partially cured ("tacky") composite material (commonly referred to as a "prepreg").
Due to the tacky nature of composite prepreg plies, handling of the plies during formation of the composite structure is difficult. One widely used method for laying up prepreg plies in a mold is by individually placing the plies by hand within the mold. The drawbacks with this type of manual lay-up procedure is that it is very labor intensive, resulting in increased manufacturing costs. For example, the prepreg composite material generally must be hand cut along guide lines to the desired shape or pattern, and hand transported to a molding station where the cut prepreg composite ply is placed or stacked in a mold cavity. Hand pressure must be applied to conform the stacked prepreg composite ply to the mold surface contour and to tack the piled prepreg composite plies together. This is a very time consuming process.
Also, since the operator must physically place the plies within the mold, there is always the chance that the plies will not be placed in the correct location. This may require post-manufacturing modifications to the component. At a minimum, it is very difficult to make two parts exactly the same.
In recent years, attempts have been made to automate the placement of the composite prepregs within the mold. These prior art placement systems use electronically controllable suction devices for lifting and placing the prepreg composite plies. However, theses automatic handling devices have had only limited success. Such devices have typically been designed to operate with a particular prepreg ply configuration, thereby limiting the flexibility of such devices to a single operational set-up. In addition, such devices did not accommodate mold configurations which had elaborate contours, such as those typically encountered in aerospace manufacturing applications.
Additionally, prior art material handling apparatus have not proven reliable for automated handling operations involving preimpregnated composite materials. The vacuum forces generated by such prior art devices have generally proven sufficient to pickup and retain preimpregnated composite materials in combination with the material handling apparatus. However, the prior art automated handling systems did not consistently and reliably release the composite prepreg plies upon termination of the vacuum forces.
To alleviate this problem, prior art material handling apparatus were modified to apply a positive pressure force against the preimpregnated composite material to disengage (blow) the composite prepreg from the handling apparatus. It was found, however, that the positive pressure forces used in these prior art devices still was not sufficient to reliably disengage the preimpregnated composite plies. Low magnitude positive pressure devices did not provide sufficient force to disengage the tacky prepreg ply from the handling apparatus. Large magnitude positive pressures, on the other hand, while generally sufficient to disengage preimpregnated composite materials from the material handling apparatus, tended to produce displacement errors when locating the ply in the mold.
Another problem with many of the prior automated handling systems was the inability to transfer more than one composite prepreg ply at a time. The design of the transfer head was such that, once a ply was picked up, the head had to transfer the ply to a mold before returning to pick up another ply. Since a pick-and-place cycle can take upwards of twenty to thirty seconds to complete, the time required to form a composite lay-up using these prior automated devices was relatively long and, consequently, expensive.
One attempt to remedy some of the deficiencies of the prior automated placement devices is disclosed in commonly-owned U.S. Pat. No. 5,183,670, which is incorporated herein by reference in its entirety. This patent discloses an automated placement system that includes a pressurized transfer head with an array of bi-functional transfer feet. Each transfer foot includes a housing with a displaceable knock-off stem. In operation, vacuum pressure is channeled through the housing to assist in retaining a composite prepreg ply to the tip of the foot. When it is desired to place the prepreg ply in a mold, positive pressure is channeled through the housing causing the knock-off stem to force the prepreg ply to disengage from the tip.
U.S. Pat. No. 5,183,670 also discloses controlling the number of feet that are activated depending on the size and shape of the composite prepreg being picked up. The desired number of feet are activated by extending the feet out from the array of feet on the transfer head and applying the vacuum to the extended feet.
Commonly-owned U.S. Pat. Nos. 5,209,804 and 5,290,386 disclose additional automated composite material handling systems which have been recently developed to address some of the problems of the prior art. These patents are also incorporated herein by reference in their entirety.
While the recent developments in automated handling systems have addressed some of the problems associated with handling composite prepreg plies, these recent systems are still do not efficiently control pickup and placement of tacky composite plies.
A need exists for an improved vacuum transfer device for accurately picking and placing composite prepreg plies in a composite manufacturing operation and which device can pickup multiple composite plies in a single cycle.